State of this notification, which can take one of the following
values: - ACTIVE: This outage notification is active. The event
could be in the past, present, or future. See start_time and
end_time for scheduling. - CANCELLED: The outage associated with
this notification was cancelled before the outage was due to start.
COMPLETED: The outage associated with this notification is
complete. Note that the versions of this enum prefixed with NS_
have been deprecated in favor of the unprefixed values.